Using custom mbox parameter variables in experience events

[#1]

I'm using targetPageParams to add a parameter into the mbox (default mbox), lets call it "MyParam". 

In my script its pulling in a querystring parameter and setting a custom mbox parameter to the value found there.

Example URL: [my URL]?TESTParam=xyz123

I know Target has seen  "MyParam" in the mbox because it offers that parameter ("MyParam") as way to build audiences off of, in "Custom" rules. 

That being said..if I wanted to create an experience that now uses the value found in MyParam on the page, let's say appending it to a text field. How can I do that? I tried using ${mbox.MyParam} in a SetHTML modification for a text area in the content section. This did not appear to render anything. 

What am I missing? Is the syntax ${mbox.MyParam} for adding that mbox variable to the page correct? Is there another way to do it? 

Thanks for any help. 

[Jchristman]

@Jchristman Can you share the mark up and the screenshot of the call to Target with the parameter being passed in?

Thanks!

[Jchristman]

Here is the script...I'm injecting it via a tag management application and pushing it out before all other load rules:
const queryString = window.location.search;
const urlParams = new URLSearchParams(queryString);
var MyParam = "";
if (urlParams.has('TESTParam')) {

MyParam= urlParams.get('TESTParam');

targetPageParams = function(){ return ["MyParam=" + MyParam]; }

; }

console.log('MyParam: ' + MyParam);

[Jchristman]

Additionally...if the syntax is correct, could it be that you can't insert an mbox parameter and pull it back out in the same page? I figured I might be able to depending on the order of the calls (insert first, mbox fill, target audience check, target experience delivered - with custom mbox param item, page render)..but I'm not sure if that is the load sequence.